1. Field of Invention
This invention pertains generally to micromachined angular rate sensors or gyroscopes, and more particularly to vibratory rate sensors utilizing dual-mass anti-phase translational movement in drive or sense modes.
2. Related Art
Many applications in automotive and consumer market require rate sensors to be insensitive to linear acceleration in all directions. In general, a rate sensor can be susceptible to linear acceleration in the drive and sense directions of the proof masses of the sensor.
For rate sensors utilizing translational drive motion, commonly used dual-mass approach with conventional coupling method, such as that disclosed in U.S. Pat. No. 5,895,850, has an in-phase linear vibration mode with resonant frequency below that of the anti-phase drive-mode. This parasitic mode allows the centroid of the proof masses to move in response to linear acceleration along the drive axis, so that the velocities of the masses are susceptible to linear acceleration in the drive direction. To reduce this susceptibility, the in-phase mode must be well suppressed such that its resonant frequency is much higher than that of anti-phase drive-mode.
Geen disclosed a mechanical coupling method in U.S. Pat. No. 5,635,638, which allows anti-phase linear movement of the two-coupled masses but resists their in-phase movement. However, the in-phase mode cannot be well suppressed due to the fact that the arcuate coupling member used is prone to bending and buckling under linear acceleration forces in the drive direction. Furthermore, no actual gyroscope device utilizing that coupling method has been disclosed.
Geen disclosed a different mechanical coupling method in U.S. Pat. No. 6,877,374 in a Z-axis gyroscope that leads to a better suppression of in-phase translational movement of the proof masses. However, this coupling structure is complex which includes multiple levers, pivots, and flexures, and each proof mass has to split into two parts that move in arcuate motion, instead of pure linear movement. The unbalanced arcuate motion of the two parts of a proof mass due to the process imperfections results in an undesired net movement of the masses in sense direction, which causes the error of the sensor.
U.S. Pat. No. 7,036,372 discloses a Z-axis gyroscope that utilizes a mechanical linkage between two masses which resists in-phase movement. Again, that linkage is complicated with requirement of multiple stiff rotation beams with inner anchor points as pivots that occupy a considerable layout area. Furthermore, the linkage must include pivots anchored to the substrate. That makes it impossible to mount the linkage on movable structure, which is often desirable in multi-axes rate sensors.
For many vibratory dual-mass gyroscopes, the rejection of linear acceleration in sense direction, which is perpendicular to drive axis, is achieved by differential processing of the electrical signals from the sense responses of the two masses. The spurious sense signals caused by linear acceleration noise can only be cancelled when the two masses and their resonant frequencies of sense modes are identical, which are impractical due to the process imperfections. And the un-cancelled residual signal becomes the error and noise in the rate signal.